Certain aspects of vehicle structures, such as the structural components of aircraft, must be made resistant to the energy typically generated by sparking and arcing resulting from transient events, such as a lightning strike. For example, structures such as aircraft fuel tanks may enclose a combustible atmosphere. Such structures must maintain integrity and cannot transmit electrical transients that might create sparks in the combustible atmosphere. Wing fuel tanks on an aircraft may have literally thousands of structural fasteners protruding into the fueled volume. A fuel tank may be defined by discrete plates joined together. Such structural fasteners and structural joints between plates must be enclosed by seals that are capable of withstanding high electrical transient currents that may result from lightning strikes, or from faults or shorts in the aircraft electrical system, without emitting an arc or spark into the tank that can be an ignition source.
Other types of structures, such as hydraulic and fuel lines on an aircraft, also must maintain their integrity and not transmit arcs or sparks into the interiors of the fuel tanks and lines. For example, hydraulic fittings are widely used in aircraft and other vehicles and applications. Such fittings may be placed over two or more tubes to be joined and mechanically swaged to make a fluid-tight joint.
With the transitioning of aircraft structural components from metal to non-metal composites, the lines of hydraulic control systems on such aircraft have been utilized as current carrying means for dissipating electrical energy from lightning strikes. The hydraulic lines also may serve as part of a current return network that is used for grounding electrical systems and discharging electrical energy from the aircraft.
One potential consequence of passing high levels of current through hydraulic lines as part of the current return network is that arcing might occur between a hydraulic line and a fitting. The FAA (Federal Aviation Administration) has recognized this risk and promulgated Federal Aviation Regulation (FAR) 25.981(a)(3) (14 C.F.R. §25.981(a)(3)) that requires any potential ignition source in a fuel tank to be sufficiently contained by redundant ignition prevention measures.
A solution to this sparking problem is described in U.S. application Ser. No. 13/167,809 filed Jun. 24, 2011 titled “Apparatus for Preventing Spark Propagation”, the contents of which are incorporated herein by reference. The apparatus described therein prevents spark propagation from a fitting to a high-risk zone by means of a cover or wrap that provides a physical barrier between the fitting and the high-risk zone.
In addition to preventing the spark from travelling from the spark location, which may include a lightning strike, to a high-risk zone, it is also desirable to eliminate the risk of sparking entirely by absorbing the energy discharged when the spark occurs. One solution for mitigating this risk is the use of a sealant, such as a sealant meeting Aerospace Material Standard (AMS) AMS3281 rev E, published by the SAE (Society of Automotive Engineers) specifications. Such sealants are electrically resistant, polysulfide-based, low-density, high-temperature resistant material that may be applied in paste form. Such sealants, such as PR-1776M Class B Low Weight Fuel Tank Sealant provided by PPG Aerospace, are used not only as a fuel tank sealant but as a sealant for other aircraft fuselage sealing applications, and as a sealant between a fitting and a cover or wrap.
Sealants meeting the AMS3281 rev E specifications are resistant to fuel and hold up in the aircraft environment. Such sealants may be used for pre-cured fastener caps that are attached to fasteners that pierce an aircraft fuel tank. When such sealants are exposed to normal temperatures, the material is pliable and able to absorb a large amount of energy. However, at colder temperatures (e.g., −60 to −80° F.) the material may become brittle and may fail when exposed to high pressure gases, such as from electrical sparking. In addition to embrittlement, the sealant, even in pliable form, behaves as an incompressible fluid. A lightning strike can cause an expanding bubble of gas which must not be allowed to create a path to the fuel tank. Sizing a seal to prevent such paths increases its size, weight, and cost.
Therefore, there is a need for a seal for joints and fittings that may function in combustible atmospheres. There is a need for seals and fittings having increased energy-absorbing qualities that provide resistance to high transient pressures within joints and attenuate sparking and electrical discharges from transient voltages, and maintain integrity and flexibility over a wide temperature range. There is also a need for such fittings to absorb the gas volume generated by a short pulse of high current, such as a lightning-induced spark.